Limit driver amount to only a certain part of a rotation
Rotations in blender are natively in radians.  To linearly map 90 to 135 degrees to 0 and 1 we can use

 (r - radians(90)) / radians(45)


will give us 0 when r = radians(90) and 1 when r = radians(135).

To be sure the end points are taken care of, (The shape key min max should do this too) and the driver expression returns 0 for angles less than 90 and 1 for angles greater than 135.

 min(max(0, (r - radians(90)) / radians(45)), 1)